Short & sweet because other reviewers have provided wonderful descriptions already: reading such a book as this is an education in itself.
Brideshead is a classic novel by a genuine master of English prose. Well-worth reading not once, but many times, to understand the depth of the story itself as well as appreciate Waugh's obvious mastery of language.
Also highly recommended is Mortimer's adaptation of the book as a mini-series starring Laurence Olivier, John Gielgud, Anthony Andrews and Jeremy Irons. It is the definitive Brideshead on film, from the opening lines spoken by Jeremy Irons (as usual, his speaking voice is flawless) to the final scene of Charles in Brideshead chapel during WWII where Charles prays "an ancient prayer, newly learned."
(There are some reviewers who've given it a low rating based on their dislike of the underlying theme of the book. Evelyn Waugh was a convert to Catholicism and his novel revolves around the characters' wandering away but ultimately back again, to faith: for the Flyte family, it is a return to their heritage (two of the most moving scenes are Lord Marchmain's death-bed conversion and Julia's painful but utterly noble decision), and for Charles Ryder (not "Simon" as a one-star critic mistakenly called him! Have you read the book, sir?), it is a newly found conviction. Hence, Book III's title "A Twitch upon the Thread" (quoting Chesterton), the thread referring to the fine, but strong pull of the Catholic faith over these individuals. If this is the book's only 'flaw', as some assert it to be, perhaps this line from a Capra film will help: for those who believe, no explanation is necessary; for those who do not, no explanation is possible. Agnostic, atheist or believer, the workings of grace is a mysterious thing.)